In recent years, rapid development of semiconductor technology has led to rapid progress of a reduction in size of semiconductor packages, the adoption of multipin, the adoption of fine pitch, minimization of electronic components and the like. That is, the semiconductor field has entered the so-called “age of high density packaging.” Regarding printed wiring boards, this has also led to a change from single side wiring to double side wiring and, in addition, the adoption of a multilayer structure and a thickness reduction (Iwata and Harazono, “Denshi Zairyo (Electronic Material),” 35 (10), 53 (1996)).
Pattern formation methods used in the formation of such wiring and circuits include: a method which comprises the steps of etching a metal layer, provided on a substrate having a layer construction of metal layer/insulating layer/metal layer, with an acidic solution, such as a ferric chloride solution, to form wirings, and, in order to provide layer-to-layer continuity, then subjecting the insulating layer in a dry state to dry etching, such as plasma etching or laser etching, or wet etching, for example, with hydrazine to remove the insulating layer to form a desired shape (Japanese Patent-Laid Open No. 164084/1994), and connecting the wirings to each other, for example, through plating or electrically conductive paste; and a method (Proceedings of the 7th Symposium of Japan Institute of Electronics Packaging, 1999) which comprises the steps of providing an insulating layer in a desired form using a photosensitive polyimide (Japanese Patent Laid-Open No. 168441/1992) or the like and then plating gaps to form wiring.
A tendency toward downsizing of electric products in recent years has led to a reduction in thickness of each layer constituting metal layer/polymeric insulating layer, and these layers each are in many cases used in a thickness of not more than 100 μm. When wiring has been formed of such thin layer, a warpage disadvantageously takes place in wiring due to a difference in coefficient of thermal expansion between the metal layer and the polymeric insulating layer.
When the thermal properties of the insulating layer and the conductor layer are known, the warpage a of this substrate can be calculated according to the following equation (Miyaaki and Miki, NITTO TECHNICAL REPORT, 35 (3), 1 (1997)).
  σ  =                    31        ⁢                                  ⁢                  E          1                ⁢                  E          2                            2        ⁢                                  ⁢                  h          ⁡                      (                                          E                1                2                            +                              14                ⁢                                                                  ⁢                                  E                  1                                ⁢                                  E                  2                  2                                            +                              E                2                2                                      )                                ⁢    Δ    ⁢                  ⁢    α    ⁢                  ⁢    Δ    ⁢                  ⁢    T  wherein
E1: modulus of the metal,
E2: modulus of the insulating layer,
Δα: difference in coefficient of thermal expansion between the metal and the insulating layer,
ΔT: temperature difference, and
h: layer thickness 1: wiring length.
According to this equation, the following two methods are considered effective for reducing the warpage of wiring:
1. a method wherein the modulus of the insulating layer is reduced; and
2. a method wherein the difference in coefficient of thermal expansion between the insulating layer and the metal wiring layer is reduced.
Regarding the wiring formation method, in the laminate used in the method for the formation of wiring through etching of a metal layer in the laminate having layer construction of first metal layer/insulating layer/second metal layer, in order to reduce the warpage of the laminate, the coefficient of thermal expansion of the metal layer should be made identical to the coefficient of thermal expansion of the insulating layer. To this end, the use of a low expansion polyimide as the insulating layer in the laminate has been proposed (U.S. Pat. No. 4,543,295 and Japanese Patent Laid-Open Nos. 18426/1980 and 25267/1977).
Since, however, the low-expansion polyimide is not generally thermoplastic, the adhesion to the metal layers is poor making it difficult to provide adhesive strength high enough to withstand practical use. It is known that, in order to overcome this problem, a thermoplastic polyimide resin or epoxy resin having good adhesion to the metal layers is used as an adhesive layer between the metal layer and the insulating layer (core layer) of the low-expansion polyimide (Japanese Patent Laid-Open No. 58428/1995).
In this case, the thickness of the low expansion core insulating layer is made larger than the thickness of the adhesive layer to avoid the appearance of warpage in the whole laminate. The thinner the adhesive layer, the better the effect of preventing warpage. Excessively small thickness of the adhesive layer, however, is detrimental to the adhesion. Further, at least when the total thickness of the upper and lower adhesive layers provided respectively on the upper side and the lower side of the core insulating layer is not more than the half of the thickness of the core insulating layer, the warpage is less likely to occur. For this reason, for commercially available laminates, the total thickness of the adhesive layers is in many cases not more than the half of the thickness of the core insulating layer (Japanese Patent Laid-Open No. 245587/1989).
In etching these materials by the wet process, however, when the two materials are different from each other in etching rate, in general, the edge shape is not straight and, in this case, the material having a lower etching rate is left. When the etching rate of the adhesive layer is lower than the etching rate of the core insulating layer, the edges of the upper and lower adhesive layers, which have sandwiched the core insulating layer therebetween, are projected. On the other hand, when the etching rate relationship is opposed to the above case, the adhesive layers are etched earlier than the core insulating layer and, in this case, the etched center is projected. The above problem makes it very difficult to perform patterning of the insulating layer by wet etching. Therefore, the insulating layer is generally patterned by the dry process using plasma or laser.
At the present time, rapid expansion of production of personal computers has also lead to increased production of hard disk drives incorporated in the personal computers. A component, in the hard disk drive, called a “suspension,” which supports a head for reading magnetism, is being shifted in its main products from one, wherein copper wiring is connected to a stainless steel plate spring, to one called a “wireless suspension” comprising copper wiring which has been connected directly to a stainless steel plate spring, from the viewpoint of coping with the size reduction.
The wireless suspension is mainly formed of a laminate of first metal layer/adhesive insulating layer/core insulating layer/adhesive insulating layer/second metal layer. An example of the laminate comprises a first metal layer of a copper alloy foil, a second metal layer of a stainless steel foil, and an insulating layer composed of a core insulating layer and an adhesive insulating layer stacked on both sides of the core insulating layer. In the wireless suspension using the laminate, since scanning on a disk being rotated at a high speed is carried out, fine vibration is applied to the member. Therefore, the adhesive strength of the wiring is very important. For this reason, satisfying severe specifications is required of wireless suspensions using the laminate.
For components called the wireless suspension, there are two production methods, that is, an additive method wherein wirings are mainly formed through plating, and a subtractive method wherein a copper foil is etched to form wirings. In the case of the subtractive method, for the above reason, plasma etching is used for patterning a polyimide as the insulating layer.
The laminate as the material before etching for wireless suspensions has hitherto been etched only by the dry process. In the dry process, in general, etching is carried out for sheet by sheet (sheet method). Therefore, disadvantageously, the productivity is low, and, in addition, sine the apparatus is expensive, the production cost is very high. On the other hand, in the wet process, a continuous product can be continuously etched. Therefore, advantageously, the productivity is high, and, in addition, the apparatus cost is low. Regarding laminates for wireless suspensions required to satisfy severe specifications, however, the wet process could not have been put to practical use for the following reason.
As compared with etching by the dry process, in the case of etching by the wet process, materials to be etched are in many cases greatly different from each other in etching rate. For example, as described above, the adhesive used as a part of the insulating layer in the laminate for wireless suspension is mainly formed of a polyimide resin from the viewpoint of the necessity of ensuring a high level of insulation reliability. As described above, imparting thermoplasticity is a general method for imparting adhesion to the polyimide resin. The introduction of a flexible structure, which can impart thermoplasticity, into the skeleton of the polyimide resin, however, leads to a tendency such that the chemical resistance is increased. Therefore, this resin is likely to be poor in suitability for etching by the wet process, and, thus, the etching rate is much lower than the etching rate of the core insulating layer. As used herein, the term “etching rate” refers to the degree of a reduction in film thickness per unit time caused by etching.
In an insulating layer formed by providing a low expansion polyimide as a core insulating layer, providing an adhesive polyimide as an adhesive insulating layer, and combining and stacking these layers on top of each other or one another to form an insulating layer formed of a plurality of layers (for example, adhesive insulating layer/core insulating layer/adhesive insulating layer or adhesive insulating layer/core insulating layer), when this insulating layer is etched by the wet process, the etchability of the adhesive insulating layer is likely to be poor because the chemical resistance of the adhesive insulating layer is likely to be high, while the core insulating layer is easily etched. Therefore, the etching of the whole insulating layer does not proceed evenly, and, thus, even etching shape cannot be disadvantageously provided.
Thus, optimizing the etching rate of the core insulating layer and the etching rate of the adhesive insulating layer in the insulating layer in the laminate for electronic circuit components, such as wireless suspensions, and, at the same time, improving the adhesion of the adhesive layer are contradictory to each other and are difficult to simultaneously realize. However, both the above requirements should be simultaneously satisfied.
FIG. 1 is a conceptual cross-sectional view showing the etching shape of a laminate wherein a laminate for a wireless suspension, having a layer construction of first metal layer/adhesive insulating layer/core insulating layer/adhesive insulating layer/second metal layer, which has hitherto been etched only by the dry process, has been etched by the wet process from the first metal layer side. In FIG. 1, numeral 1 designates a core insulating layer formed of a low expansion polyimide resin. A first metal layer 2 is provided on one side of the core insulating layer 1 through an adhesive insulting layer 4. A second metal layer 3 is provided on the other side of the core insulating layer 1 through the adhesive insulating layer 4. As shown in FIG. 1, in the wet process, since, the etching rate of the adhesive insulating layer 4 is lower than that of the core insulating layer 1, the adhesive insulating layer 4 in its portion remaining unetched is left in the form of the eaves of a roof. Therefore, in the whole insulating layer, the etching shape is uneven.
Up to now, any polyimide resin, used for an adhesive insulating layer in a laminate for a wireless suspension having the above layer construction, which is suitable for etching by the wet process and has excellent adhesion, is not known.